Transmission line windows



Oct. 9, 1962 A. ELDREDGE TRANSMISSION LINE wmmows FIGZ).

INVENTOR:

ARNOLD L. ELDREDGE HIS ATTORNEY.

United States Patent Ofiice Patented Oct. 9, 1962 3,058,073 TRANSMHSSIUN LHNE WRNDGWS Arnold L. Eldredge, Woodside, Calif., assignor to General Electric Company, a corporation of New York Filed Dec. 1, 1959, Ser. No. 856,611 4 Claims. (til. 333-08) This invention relates to barriers afor pressurized or evacuated waveguides or coaxial lines. That is, the invention relates to barriers which separate two regions in a waveguide or Waveguide system and provide for the transmission of electromagnetic waves between the two regions. Barriers or seals of the type under consideration are especially useful in conjunction with a transition be tween different sizes of waveguide.

An example of waveguide windows or barriers of the type under consideration is found in the input and output waveguide connections of an electronic tube which operates at microwave frequencies. In such applications electromagnetic energy must be transmitted between the evacuated interior of the tube envelope and waveguide systems which may be maintained at atmospheric pressure. Other examples of such windows are found in applications where a barrier is required to prevent the escape of gas from a pressurized system or where a barrier is required to contain a cooling or insulating fluid in a waveguiding system. These barriers, which are called microwave windows, have taken various forms using a variety of metals and dielectrics.

The use of microwave windows introduces a number of electrical problems which would not otherwise be present. The electrical problems include the introduction of reflections at the dielectric barrier due to discontinuity in the medium in which the electromagnetic waves must travel, voltage breakdown in the presence of high electric and magnetic fields in the area of the dielectric barrier and the dissipation of microwave power within the dielectric material. The mechanical problems encountered include difliculty in providing a leakproof mechanical design which can withstand the elevated temperatures required for processing and operation (may be as high as 600 C.), has the mechanical strength to maintain line dimensions at the correct values and is capable of reproduction in quantity with uniformity in dimensions and properties.

The electrical problems involved may best be understood by considering a particular application for micro wave windows of the type under consideration. As an example consider a window for the transmission of microwave energy from a high power source such as a klystron to an external waveguide system. An output window for V,

such generators should be capable of transmitting the generated power to the waveguide system with a minimum of reflection and absorption of energy. Power absorbed at the window or reflected by the window is lost and reflected energy may severely damage the source. Power absorption in a window of a given geometry is a function of'properties of the dielectric material and may be determined by selecting the material. Reflections in a transmission line are caused by discontinuities along the length of the line which represent abrupt discontinuities in impedance of the line. Such discontinuity may be caused by a dielectric barrier positioned in the transmission line or it may be caused by a change in the size of the transmission line or both.

In the application under consideration it is common to utilize a small transmission line to receive power directly from the generator and provide a transition to a larger transmission line in the waveguide system. The reason for this is that there is frequently a space limitation inyolved in getting into and out of the microwave source which dictates the use of small waveguide. The ability of the small waveguide to withstand the relatively high voltages without voltage breakdowns is insured by evacuating the waveguide along with the vacuum tube. The waveguide system which the source supplies generally cannot conveniently be presurized or evacuated. Therefore, the external waveguide system must have larger dimensions than the evacuated guide if voltage breakdown is to be avoided.

The problem of providing such a transition between the different waveguide systems is not a new one and involves matching the impedance between the waveguide on opposite sides of the window or barrier including matching the impedance in such a manner that the barrier itself does not represent an abrupt reflective discontinuity. The matching problem is not diflicult for a particular frequency or a limited frequency range but many of the generators in use are designed to operate over a wide range of frequencies and it is difficult to obtain an impedance match which is effective over the desired frequency. range. For example, a typical problem is to obtain an impedance match between waveguide systems which operate over a frequency range of 1500 megacycles per second centered around a frequency such as 2500 megacycles per second, that is, over a frequency range from 1750 megacycles to 3250 megacycles per second. The solution to the problem usually involves a process known as tapering the impedance. That is, the proportions of the transition are made such that the impedance changes very gradually (is tapered) so that reflections are maintained at a minimum. When considering any individual frequency in the range, the impedance match generally represents an engineering compromise. However, the window and transition under consideration gives a very good impedance match over an extremely broad range of frequencies.

If an arc over or voltage breakdown occurs in the area of the window the problems mentioned with respect to impedance mismatches occur but the situation is a great deal more acute. That is to say that if the design of the transition and barrier is such as to allow an are over some power may be transmitted but most of the incident power is consumed in the discharge and reflected. Further, if a breakdown occurs in the area of the barrier the barrier is likely to be ruptured.

The mechanical problems are easier to understand than the electrical problems. For example, it is easy to see that if the design of the Window and transition is not such as to permit a vacuum tight joint the barrier is ineffective. Also, if the mechanical strength is not sufficient to maintain the dimensions both under normal operating conditions and at the elevated temperatures required for processing, the advantages of a careful design are lost. Further, unless the design is capable of production and reproduction in quantity with uniformity in dimensions and properties the usefuliness is reduced considerably.

Accordingly it is an object of this invention to provide a microwave window or barrier and transition to separate two regions of a transmission line system which presents a simple strong mechanical vacuum tight struc ture and which produces few reflections over an extremely wide band of frequencies.

Briefly stated in accordance with one aspect of this invention a transition and microwave window is formed between coaxial conductors of different sizes by tapering the inner and outer conductors of a coaxial transmission line section to provide for impedance matching and providing a dielectric barrier within the transition section which seals the section and has a contour that conforms as closely as possible to a contour of equal electric field strength.

The novel features which are believed to be characteristic of the invention are set forth in the appended claims. The invention itself, however, both as to its organization and method of operation togther with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

FIGURE 1 is a broken away perspective view illustrating a window and transition constructed in accordance with the present invention;

FIGURE 2 is a side elevation with a partial central, vertical, longitudinal section taken through the window and transition illustrated in FIGURE 1; and

FIGURE 3 is a central, vertical, longitudinal section taken through a window and transition similar to the one illustrated in FIGURES l and 2 but showing a slightly different embodiment of the invention.

Referring specifically to FIGURES 1 and 2, two coaxial transmission lines and 11 are joined together by transition and window section 12. For simplicity the systems joined by transmission lines 10 and 11 and transition 12 are not illustrated. However, it is noted that one of the transmission lines (10) is smaller in diameter than the other.

The smaller coaxial transmission line 10 consists of an outer cylindrical or tubular conductor 13 and an inner conductor which is also illustrated as being tubular. The inner conductor 14 is coaxially disposed with respect to the outer tubular conductor 13. The second coaxial transmission line 11, as illustrated, has an outer tubular conductor 15 which is larger in diameter than the outer tubular conductor 13 of the first coaxial transmission line and an inner tubular conductor 16 having substantially the same diameter as the outer tubular conductor 13 of the first coaxial transmission line 10. In one application of the transition and window section 12, one of the transmission line systems is evacuated whereas the other system is maintained at atmospheric pressure. The relative pressure of the two coaxial transmission lines is not germane to this invention just as the systems joined are not germane to this particular invention.

In order to join the two transmission lines 10 and 11 having diiferent sizes and provide a continuous conductive path or waveguide, the transition section is provided with a generally frusto-conical shaped conductive nose portion 17 which extends from the end of the outer conductor 15 of the larger transmission line 11 inwardly to join the outer periphery of the outer tubular conductor 13 of the smaller coaxial transmission line 10. The opposite ends of the nose portion 17 are illustrated as being extensions of the outer conductors 13 and 15 of the two transmission lines 10 and 11. However, it is obvious that the nose portion 17 may be a separate conductive section with its opposite ends sealed, as by brazing, to the outer conductors 13 and 15 of the two transmission lines 10 and 11 to form vacuum tight rigid mechanical joints. The mechanical and electrical connection is made between the inner conductors 14 and 16 of the two coaxial transmission lines 10 and 11 by a generally frusto-conical nose cone 18 which corresponds in shape and function to the nose portion 17 of the transition 12. That is to say that the nose cone 18 tapers inwardly from the inner conductor 16 of the larger coaxial transmission line 11 to join the smaller inner conductor 14 of the smaller transmission line 10. As in the case of the outer nose portion 17 and conductors 13 and 15, the inner nose cone 18 and conductors 14 and 16 are illustrated as comprising a unitary conductive member. However, again it is clear that the nose portion 18 may be a separate conductive member with its opposite ends sealed to the adjacent conductors to form vacuum tight and mechanically rigid joints.

Thus the two outer waveguide conductors are joined by a frusto-conical nose portion 17 and the two inner conductors are joined by a similar frusto-conical nose cone 18 which is coaxially positioned inside the larger nose portion in such a manner that a substantially conical waveguide passage is defined between the two cones. Without the addition of a window the structure thus far described provides an excellent transition between coaxial waveguides having difierent diameters. If the two coaxial transmission lines which are joined by the transition have the same characteristic impedance, the relative angles of the sides of the conductive cones 17 and 18 may be selected so that the characteristic impedance is essentially constant throughout the transition for an extremely wide range of frequencies. If the impedance of the two coaxial transmission lines is different, that is, if the ratio of the inner to outer diameter of the conductors of the lines is different, the relative angles of the sides of the conductive cones 17 and 18 may be selected so that the impedance transition from one line to the other is gradual and reflections are kept to a minimum.

The vacuum tight barrier between the two transmission lines 10 and 11 is provided by a cylindrical ceramic dielectric microwave Window 20. A typical window material is a commercially available high alumina ceramic or glass. The cylindrical window, as illustrated, has approximately the same diameter as the outer conductor 13 of the smaller coaxial transmission line 10 and the inner conductor 16 of the larger transmission line 11. The cylindrical window 20 is positioned coaxially with respect to both transmission lines 10 and 11 and extends between a retaining shoulder 21 on the nose portion 17 which connects the outer conductors 13 and 15 and a similar retaining shoulder 22 on the nose section 18 which connects the inner conductors 14 and 16 of the two transmission lines 10 and 11. Thus, in the vertical, longitudinal section of the transition 12, as illustrated in FIG- URE 2, the cylindrical ceramic window 20 appears to form a continuous cylindrical member with the outer tubular conductor 13 of the smaller coaxial line 10 and the inner tubular conductor 16 of the larger transmission line. The opposite ends of the cylindrical barrier or window 20 are sealed at the shoulders 21 and 22 on the outer and inner conductors 13 and 16 respectively to form vacuum tight seals. Thus, the waveguide area between the outer and inner conductors 13 and 14 of the smaller coaxial transmission line 10 is sealed from the waveguide area between the outer and inner conductors 15 and 16 of the larger transmission line 11 by a microwave window 20. The window 20 provides for transmission of electromagnetic waves from one guide to the other and at the same time provides a means for maintaming a pressure dilferential therebetween. For example, the area inside the smaller coaxial transmission line 10 may be evacuated and the area between the conductors 15 and 16 of the larger transmission line 11 may be at atmospheric pressure or the situation may be reversed if desired or necessary. As previously described the angle of taper or flare of the sides of the inner cone section 18 relative to the corresponding angle of the outer conical nose cone 17 may be selected so that reflections at the window and transition may be minimized in the desired frequency range.

A further embodiment of the present invention is illustrated in FIGURE 3. In general the embodiment illustrated in FIGURE 3 contains the same elements as the embodiment illustrated in FIGURES 1 and 2. To simplify the description, therefore, corresponding elements in the figures are given the same reference numerals. That is, the two coaxial transmission lines are again given the reference numerals 10 and 11 and their inner and outer conductors are again given the same numerals. Specific elements of the transition illustrated in FIGURE 3 are given reference numerals which are different from their counterparts in the previous figures since this transition has constructional features which are slightly difierent.

In this embodiment, the outer conductor 15 of the large coaxial transmission line 11 is turned down at its end as by spinning to form a frusto-conical nose portion 2.6. The end of the nose thus formed fits snugly around the outer periphery of the outer conductor 13 of the smaller coaxial transmission line 10. A rigid vacuum tight joint is formed here by some means such as brazing. The inner conductors 14 and 16 are connected by extend ing the inner conductor 14 of the smaller transmission line beyond the outer end of the outer conductor 13, flaring its outer end outwardly to form. a substantially frusto-conical nose section 27 which has a base diameter that fits snugly into the inner conductor 16 of the larger transmission line 11. A rigid and vacuum tight joint is formed at the line of contact by any convenient means such as brazing.

The primary difference between the tWo embodiments illustrated resides in the barrier or wind-ow which forms the seal that permits the two coaxial transmission lines to be operated at different pressures. This barrier 25 is also cylindrical and, as shown, extends between the outer conductor 13 of the smaller transmission line and the inner conductor 16 of the larger transmission line. The joints between the barrier 25 and these conductors (13 and 16) is vacuum tight.

As illustrated in FIGURE 3 the barrier is a composite structure which includes two cylindrical dielectric win- 28 and 30 and a cylindrical conductive member 31. These three members are positioned end to end on a common axis so that the two windows 28 and 30 are separated by the conductive cylinder 31 and vacuum tight brazes are formed between them so that the barrier 25 is one unitary structure. The range of frequencies accepted or passed by the structure is extended by utilizing the two cellular resonant windows 28 and 30 each resonant at different frequencies. This principle may be extended by using more than two cellular resonant windows, each resonant at a different frequency.

In the two embodiments illustrated the barrier or window portion in the transition between the two coaxial transmission lines 10 and 11 is of approximately the same diameter as the outer conductor of the smaller transmission line and the inner conductor of the large transmission line. A consideration of the broader aspects of the invention will make it obvious that this condition does not have to obtain and the cylindrical barrier could be applied in the transition anywhere along the tapered portion. The important feature is that the position of the window or barrier conforms as closely as possible to a contour of equal field strength in the transition. The value of the field contour is dictated by voltage breakdown and average heating considerations and depends upon the operating characteristics of the equipment and the type of ma terials being used.

The configurations proposed have a number of particular advantages. For example, the value of dielectric strength required for the window material is a minimum since no areas of high field difierential exist and loss in the window is uniform, hence temperature gradients are minimized. Also the required ceramic is kept to a minimum diameter, has a high mechanical strength, and produces a minimum of reflection of electromagnetic wave at the barrier.

While particular embodiments of the invention have been shown it will of course be understood that the invention is not limited thereto since many modifications may be made. It is contemplated that the intended claims will cover any such modifications as fall within the true spirit and scope of the invention.

What I claim is new and desire to secure by Letters Patent of the United States is:

1. A transition and window for connecting two coaxial transmission line systems and providing a vacuum tight joint therebetween which is capable of transmitting electromagnetic energy comprising a pair of frusto-conical shaped conductive sections, at least a first one of said sections being hollow, the other one of said sections coyaxially positioned relative to said first section in such a manner that the conical sides of both sections slope in the same direction and are spaced apart, and a substantially cylindrical dielectric means positioned concentrically between said conductive sections extending therebetween and sealed thereto to provide rigid vacuum tight mechanical connections; the minimum diameter of said first section, the maximum diameter of the other one of said sections, and the diameter of said dielectric means being substantially equal.

2. In a transition and window assembly for providing a vacuum tight barrier between two coaxial waveguide systems which permits transmission of electromagnetic waves over a broad range of microwave frequencies, a first hollow conductive waveguide means having the internal configuration of the frustum of a cone, a second conductive waveguide means having the external con figuration of a frustum of a cone, said first and second conductive means being proportioned such that the base of the cone of said second waveguide means is smaller in diameter than the base of the cone defined by said first Waveguide means, said first and second conductive members being coaxially positioned and spaced apart in such a manner that a substantially conical waveguiding space is defined therebetween, and a substantially cylindrical dielectric member coaxially positioned with respect to said first and second waveguide means and within said conical waveguiding space therebetween, opposite ends of said dielectric member being secured to opposite ones of said first and second waveguide means thereby to form a rigid mechanical assembly; the minimum diameter of said first waveguide means, the maximum diameter of said second waveguide means, and the diameter of said dielectric member being substantially equal.

3. In combination in a microwave transition and transmission line seal assembly for insertion between two coaxial waveguide systems operating at different pressures and utilizing coaxial transmission lines with different outer diameters; a first coaxial waveguide means for connection to one waveguide system, said first waveguide means comprising a hollow cylindrical outer conductor and a cylindrical inner conductor concentrically disposed therein to define a hollow cylindrical waveguiding space therebetween, a second coaxial waveguide means for connection to a second waveguide system, said second waveguide means comprising a hollow cylindrical outer conductor of smaller diameter than the said outer conductor of said first coaxial waveguide means and a cylindrical inner conductor concentrically disposed therein to define a hollow cylindrical waveguiding space therebetween; and a transition section between said first and second waveguide means comprising two substantially frusto-conical shaped conductive transition members concentrically positioned and spaced apart, one of said transition members extending between the said outer conductors of said first and second waveguide means and forming a conductive mechanical connection therebetween, the other one of said transition members extending between the said inner conductors of said first and second waveguide means and forming a conductive mechanical connection therebetween, and a cylindrical window means including at least one cylindrical dielectric member concentrically positioned in said transition section and extending between said transition members to form a vacuum tight barrier which is substantially transparent to electromagnetic energy in a broad frequency range; said inner conductor of the first waveguide means, said outer conductor of the second waveguide means, and said dielectric member having substantially equal diameter.

4. In combination in a microwave transition and transmission line seal assembly for insertion between two coaxial waveguide systems operating at .difierent pressures and utilizing coaxial transmission lines with different outer diameters; a first coaxial waveguide means for connection to one waveguide system, said first waveguide means comprising a hollow cylindrical outer conductor and a cylindrical inner conductor concentrically disposed therein to define a hollow cylindrical waveguiding space therebetween, a second coaxial waveguide means for connection to a second waveguide system, said second waveguide means comprising a hollow cylindrical outer conductor of smaller diameter than the said outer conductor of said first coaxial Waveguide means and a cylindrical inner conductor concentrically disposed therein to define a hollow cylindrical waveguilding space therebetween; and a transition section between said first and second waveguide means comprising two substantially frusto-conical shaped conductive transition members concentrically positioned and spaced apart, one of said transition members extending between the said outer conductors of said first and second waveguide means and forming a conductive mechanical connection therebetween, the other one of said transition members extending between the said inner conductors of said first and second waveguide means and forming a conductive mechanical connection therebetween, and a composite cylindrical window and barrier means positioned in said transition section and extending between said transition members to form a vacuum tight barrier, said composite cylindrical window and barrier means including at least two coaxially disposed cylindrical dielectric members joined by a cylindrical conductive member; said inner conductor of the first waveguide means, said outer conductor of the second waveguide means, and said composite cylindrical window and barrier means having substantially equal diameters.

References Cited in the file of this patent UNITED STATES PATENTS 1,841,473 Green Jan. 19, 1932 2,453,759 Robinson Nov. 16, 1948 2,922,127 Dench Jan. 19, 1960 FOREIGN PATENTS 1,000,479 Germany Jan. 10, 1957 1,068,324 Germany Nov. 5, 1959 

